1. Field of the Disclosure
This invention pertains to a method for the preparation of composite printing forms, and particularly for the preparation of composite printing forms for use in relief printing.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials including corrugated carton boxes, cardboard boxes, continuous web of paper, and continuous web of plastic films. Flexographic printing plates are a form of relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions that typically include an elastomeric binder, at least one monomer, and a photoinitiator, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element.
Corrugated boxes and other, relatively large objects that are printed using relief image printing plates often bear actual printing on only a small portion of their total surface area. Those skilled in the art often print relatively large objects with composite printing plates that are prepared by mounting a plurality of relief image printing plates on a common carrier sheet. The individual plates, however, are mounted only on those portions of the carrier that correspond to the portions of the object that actually need to be printed. However, this system for mounting constituent relief image plates is laborious and requires careful adhesion of the plates to the carrier while assuring accurate registration on press for high quality printing and multi-color reproduction. For multi-color reproduction, wherein a single plate is used for printing each of the individual colors, accurate registration of the plates with respect to one another is particularly crucial.
Other methods of preparing composite printing plates have been suggested, such as in U.S. Pat. Nos. 5,846,691; 6,312,871; 6,312,872; 6,399,281; and 6,472,121, which do not require precise registration of constituent photocurable elements. Generally the methods involve disposing at least one photocurable element upon a surface of a carrier in approximate register and then transferring a computer generated negative to a surface of the elements. The methods include transferring registration information of any visually perceptible modification of the printing element, i.e., carrier, that is intended to reflect positioning of the photocurable elements. The negative or mask may be generated by jetting ink onto the surface of the element or by exposing with laser radiation to selectively remove a radiation opaque layer from the surface of the element.
Another way to print corrugated board and other large objects is to prepare a single relief image plate having a surface area corresponding to the total surface area of the object. Since only portion of the object's surface needs to be printed, however, only a portion of the relief image plate will actually be used for ink transfer. The remainder of the plate will be unused, and is removed by washout processing at the same time as the portion/s of the relief image is being formed. However, removal of the unused portion places extreme stress on the washout apparatus and on the relief image plate. As such, large unused portions, i.e., unimaged portions, are cut away prior to treating.
It is desirable to assure the accurate positioning of the relief printing form or forms when mounting onto the print cylinder or onto the carrier in order for the printed image/s on the substrate to be registered. The relief printing form should be positioned on the cylinder such that the printing is parallel to the axis of the printing cylinder, that is, not skewed. In multicolor printing, the relief printing form for each color being printed should be aligned so that the different color printed images are registered with each other. Registration errors give rise to superimposed colors, spaces with no color, color shifts, and/or degraded image detail.
Even with these prior methods, there remains a need in the art for alternative processes for preparing composite printing forms. In particular, there remains a need for alternative, easy to use, process for accurate registration of the constituent relief image printing plates on the composite printing form.